Superconducting intermetallic

Figure 10 Unit cell of the CrsSi (cP8, A15) type superconducting intermetallic NbsSn showing interpenetrating, orthogonal Nh atom chains. (Ref. 48. Reproduced hy courtesy of W.L. Johnson)...

Frequently, NMR has been applied for the study of simple superconducting intermetallic compounds. We refer to the review of MacLaughlin (1976) for a general introduction. Occasionally, NMR has also been applied to tackle more complicated systems, like heavy-fermion superconductors (sect. 5.5.1) or systems, where the eventual coexistence between magnetism and superconductivity was of interest (sect. 5.4.1). NMR has made important contributions to a better understanding of the high-Tj, oxide compounds, as well. The latter, growing, field of activity has to be covered in a future volume of this Handbook. 

Nevertheless deviations from eq. (9.19) have been observed for the intermetallic compound Auln2 [108,109] and for T1 [110,111], Requirements for the validity of eq. (9.19) are the absence of changing internal fields due to nuclear magnetic or electronic magnetic ordering in the relevant temperature range, the absence of nuclear electronic quadrupole interactions and no superconductive transition. 

As mentioned above in the intermetallic Section, beta-tungsten, which is chemically WsO, is the prototype of the A-15 structure. The interest in WsO, or Ws01 x, is not only structural but is also based on the fact that this material is superconducting Further surprising is that, for the first time, this oxide superconductor has a higher transition temperature than that of the metal itself. Pure tungsten metal has a Tc of 15.4 mK, whereas the oxide WsO has a reported Tc of 3.35 K. Other oxide compounds such as CrsO and "MosO", which are isostructural with WsO, do not superconduct above 1.02 K. 

With the discovery and disclosure of these events in the area of "High Tc Superconductivity", hundreds, if not thousands, of scientists actively became involved in research on these new materials. Newer materials and higher Tc s soon followed. The competition was fierce and the progress through 1987 and 1988 was moving at a rapid pace with numerous important discoveries. To date, the highest Tc is in the range of 110-125 K, some five times that obtained in 1973 on the revolutionary (A-15) intermetallic materials. These new copper -oxide systems, many of which will be described in detail by other contributors to this book, are presented in Table 10. 

Perhaps the Mg intermetallic compound which attracted most interest in the past 10 years is MgB2. The reason for this lies in its superconductivity below 39 K, which is an unusually high value for a phonon-mediated... 

Compound Semiconductors. The niobium-based superconducting compounds lead us naturally into another use for intermetallics—namely, semiconductors. This topic, too, was introduced earlier in this chapter (cf. Section 6.1.1.4 and 6.1.1.5), and we shall build upon those principles here to describe the semiconducting properties of compounds, ceramics, and glasses. The classification of intermetallics as ceramics... 

Bismuthides. Many intermetallic compounds of bismuth with alkali metals and alkaline earth metals have the expected formulas M3Bi and M3Bi2, respectively. These compounds are not salt ike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi3 . Both the alkali and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particulady noteworthy as having extremely short bond distances between the alkali metal atoms. 

Based on their difference in reacting to magnetic flux, superconducting material can be differentiated into Type-I and Type-II. The Type-11 superconductors are usually associated with intermetallic compounds instead of elements and their superconductivity is not easily affected even with a high magnetic field. 

Superconductivity has been found in metallic elements and intermetallic compounds and within their solid-solution-range. But, superconductivity has not been found in an alloy with an arbitrary composition. 

These, therefore, constitute the guidelines for finding superconductors or how to raise the superconducting temperature. Since Covalon conduction is a nucleus to superconductivity and covalent bond is a poor conductor at room temperature, a good conductor at room temperature implies a poor covalent bond and therefore will not be a superconductor or will be a poor superconductor at best at low temperature. Inasmuch as a good covalent bond can come from compound formation, good superconductors, particularly Type-II, shall be expected to come from intermetallic compounds or special type of ceramic oxides and nitrides. 

P. Thalmeier and G. Zwicknagl, Unconventional superconductivity and magnetism in lanthanide and actinide intermetallic compounds 135... 

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